The neuroendocrine hormone prolactin (PRL) is an important growth and differentiation factor for the human breast. The mediation of these effects of PRL on breast issues occurs through the prolactin receptor (PRLr), a Type I transmembrane receptor and member of the cytokine receptor superfamily. Within the breast, four structurally and functionally distinct PRLr isoforms (the long, intermediate, deltaS1, and PRLBP) are expressed. The extracellular binding of ligand by the PRLr isoforms initially induces receptor dimerization and phosphorylation that activates intracellular PRLr-associated signaling proteins The triggering of these PRLr associated signaling networks results in the enhanced growth and motility of human breast cancer cells. Our laboratory has demonstrated that tyrosine phosphorylation of the PRLr is necessary for these actions. PRLr action is also modulated by stimulation with extracellular matrix, indicating the presence of signaling intermediaries between the integrins and the PRLr. We have recently demonstrated that the complex of the transmembrane protein SHPS1 and the protein tyrosine phosphatases (PTP) SHP1 and SHP associate with the deltaS1 PRLr and undergo tyrosine phosphorylation following PRL stimulation. Additional data have also revealed that the SHPS1/SHP1/SHP2 complex, in turn, can modulate the signaling and function of associated receptors. Given these findings, it is the central hypotheses of this proposal that phosphorylation of the PRLr isoforms and their interaction with the SHPS1/SHP1/SHP2 complex contributes to the in vitro motility and in vivo progression of human breast cancer. This hypothesis will be tested by three specific aims using tissue culture and xenograft models of human breast cancer. First, the mechanism and functional significance of PRLr isoform phosphorylation will be examined through molecular approaches. Second, the phosphorylation, association, and role of the SHPS1/SHP1/SHP2 complex during the PRLr signaling will be assessed by over-expression of wild type and mutant forms of these proteins. Third, the role of the phosphorylated PRLr isoforms and the SHPS/SHP1/SHP2 complex to breast cancer motility and metastasis will be evaluated. These studies will provide insight into the function of the newly discovered PRLr isoform and the associated SHPS1 complex, further mapping the function of PRL within the breast. Such structure/function analysis of the PRLr isoforms may ultimately provide the basis for novel therapeutic strategies aimed at interrupting the function of the PRL/PRLr complex in human breast cancer.